With respect to the method for producing a dialkyl carbonate and a diol by reacting a cyclic carbonate with an aliphatic monohydric alcohol, various proposals have been made. Most of those proposals relate to the development of catalysts for the above reaction. Examples of such catalysts include alkali metals or basic compounds containing alkali metals (see Patent Documents 1 and 2), tertiary aliphatic amines (see Patent Document 3), thallium compounds (see Patent Document 4), tin alkoxides (see Patent Document 5), alkoxides of zinc, aluminum and titanium (see Patent Document 6), a mixture of a Lewis acid with a nitrogen-containing organic base (see Patent Document 7), phosphine compounds (see Patent Document 8), quaternary phosphonium salts (see Patent Document 9), cyclic amidines (see Patent Document 10), compounds of zirconium, titanium and tin (see Patent Document 11), a solid, strongly basic anion-exchanger containing a quaternary ammonium group (see Patent Document 12), a solid catalyst selected from the group consisting of a tertiary amine- or quaternary ammonium group-containing ion-exchange resin, a strongly acidic or a weakly acidic ion-exchange resin, a silica impregnated with a silicate of an alkali metal or an alkaline earth metal, and a zeolite exchanged with ammonium ion (Patent Document 13), and a homogeneous catalyst selected from the group consisting of tertiary phosphine, tertiary arsine, tertiary stibine, a divalent sulfur compound and a selenium compound (see Patent Document 14).
With respect to the method for conducting the above-mentioned reaction between a cyclic carbonate and a diol, the below-mentioned four types of methods (1) to (4) have conventionally been proposed. Hereinbelow, explanation is made with respect to such methods (1) to (4), taking as an example the production of dimethyl carbonate and ethylene glycol by the reaction between ethylene carbonate and methanol, which is a representative example of reactions between cyclic carbonates and diols.
First method (hereinafter referred to as “method (1)”) is a completely batchwise method in which ethylene carbonate, methanol and a catalyst are fed to an autoclave as a batchwise reaction vessel, and a reaction is performed at a reaction temperature higher than the boiling point of methanol under pressure for a predetermined period of time (see Patent Documents 1, 2 and 5 to 9).
Second method (hereinafter referred to as “method (2)”) is a batchwise method, using an apparatus comprising a reaction vessel provided at an upper portion thereof with a distillation column, in which ethylene carbonate, methanol and a catalyst are fed to the reaction vessel, and a reaction is performed by heating the contents of the reaction vessel to a predetermined temperature. In this method, in order to compensate for the methanol distilled as an azeotropic mixture of the methanol and the produced dimethyl carbonate, the continuous or batchwise addition of supplemental methanol to the reaction vessel is optionally conducted. However, irrespective of whether or not such an addition of supplemental methanol is conducted, the reaction per se is performed only in a batch-type reaction vessel. That is, in this method, the reaction is batchwise performed under reflux for a prolonged period of time as long as 3 to 20 odd hours. (Patent Documents 15, 3 and 4).
Third method (hereinafter referred to as “method (3)”) is a liquid flow method in which a solution of ethylene carbonate in methanol is continuously fed to a tubular reactor to perform a reaction at a predetermined reaction temperature in the tubular reactor, and the resultant reaction mixture in a liquid form containing the unreacted materials (i.e., ethylene carbonate and methanol) and the reaction products (i.e., dimethyl carbonate and ethylene glycol) is continuously withdrawn through an outlet of the reactor. This method has conventionally been conducted in two different manners in accordance with the two types of catalyst used. That is, one of the manners consists in passing a mixture of a solution of ethylene carbonate in methanol and a solution of a homogenous catalyst in a solvent through a tubular reactor to perform a reaction, thereby obtaining a reaction mixture, and separating the catalyst from the obtained reaction mixture (see Patent Documents 11 and 14). The other manner consists in performing the reaction in a tubular reactor having a heterogeneous catalyst securely placed therein (see Patent Documents 12 and 13). Since the reaction between ethylene carbonate and methanol to produce dimethyl carbonate and ethylene glycol is an equilibrium reaction, the flow method using a tubular reactor cannot achieve a higher conversion of ethylene carbonate than the conversion of ethylene carbonate at the equilibrium state of reaction (the latter conversion depends on the composition ratio of the feedstocks fed to the reactor and the reaction temperature). For example, in Example 1 of Patent Document 11 which is directed to a continuous flow reaction method using a tubular reactor and wherein the flow reaction is conducted at 130° C. using a feedstock mixture having a methanol/ethylene carbonate molar ratio of 4/1, the conversion of ethylene carbonate is only 25%. This means that large amounts of unreacted ethylene carbonate and unreacted methanol, which are contained in the reaction mixture, need to be separated and recovered, which in turn are recycled to the reactor. Actually, in the method disclosed in Patent Document 13, various apparatuses are used for the separation, purification, recovery and recycling of the unreacted compounds.
Fourth method (hereinafter referred to as “method (4)”) is a reactive distillation method in which each of ethylene carbonate and methanol is continuously fed to a multi-stage distillation column to perform a reaction in a plurality of stages of the distillation column in the presence of a catalyst, while continuously effecting separation between the produced dimethyl carbonate and the produced ethylene glycol (Patent Documents 16 to 19).
Thus, the conventional methods for producing a dialkyl carbonate and a diol by reacting a cyclic carbonate with an aliphatic monohydric alcohol, can be classified into the following four methods:
(1) A completely batchwise method;
(2) A batchwise method using a reaction vessel provided at an upper portion thereof with a distillation column;
(3) A liquid flow method using a tubular reactor; and
(4) A reactive distillation method.
However, the above-mentioned conventional methods (1) to (4) have their respective problems as described below.
In the case of each of the completely batchwise method (1) and the flow method (3) using a tubular reactor, the maximum conversion of a cyclic carbonate depends on the composition ratio of the feedstocks fed to the reactor and the reaction temperature. Therefore, it is impossible to convert all of the feedstocks into the products and the conversion of the cyclic carbonate becomes low. Further, in the batchwise method (2), for improving the conversion of a cyclic carbonate, the produced dialkyl carbonate must be removed using a largely excess amount of an aliphatic monohydric alcohol, and a long reaction time is needed.
In the case of the reactive distillation method (4), it is possible to perform a reaction with high conversion, as compared to the conversions in methods (1), (2) and (3). However, needless to say, even in the case of method (4), production of a dialkyl carbonate and a diol is performed by a reversible, equilibrium reaction. Accordingly, even when it is possible to achieve a substantially 100% conversion of a cyclic carbonate by method (4), it is impossible to prevent a trace amount of the cyclic carbonate from remaining unreacted in a produced diol. Therefore, for obtaining a high purity diol by method (4), in general, it is necessary to separate the cyclic carbonate from the diol by performing a distillation under strictly controlled conditions. In Patent Document 20, it is attempted to solve this problem by hydrolyzing the unreacted cyclic carbonate to convert it into a diol. Further, in Patent Document 21, it is attempted to solve this problem by reacting the unreacted cyclic carbonate with a diol to convert it into an ether.
In addition, there have been proposed methods in which water or water having an oxygen content of not more than 100 ppm is introduced into the process for distillation purification of a diol, to thereby obtain a high purity diol having high UV transmission or low aldehyde content (see Patent Documents 22 and 23).
In the method for producing a dialkyl carbonate and a diol by reacting a cyclic carbonate with an aliphatic monohydric alcohol, the presence of a carbonate ether described in the pre-sent invention was not known in the art. The present inventors have for the first time found that a dialkyl carbonate obtained by the above-mentioned method contains a carbonate ether and that when a dialkyl carbonate contains a carbonate ether in an amount exceeding a specific level, the reaction using such a dialkyl carbonate as a raw material poses various problems. For example, it has become apparent that when a transesterification aromatic carbonate is produced from such a conventional dialkyl carbonate and phenol, the produced aromatic carbonate will contain impurities.
As can be understood from the above, no method has heretofore been proposed for producing a dialkyl carbonate and a diol from a cyclic carbonate and an aliphatic monohydric alcohol, wherein the produced dialkyl carbonate contains a carbonate ether only in a content which is reduced to a specific low range.    Patent Document 1: U.S. Pat. No. 3,642,858    Patent Document 2: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 54-48715 (corresponding to U.S. Pat. No. 4,181,676)    Patent Document 3: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 51-122025 (corresponding to U.S. Pat. No. 4,062,884)    Patent Document 4: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 54-48716 (corresponding to U.S. Pat. No. 4,307,032)    Patent Document 5: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 54-63023    Patent Document 6: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 54-148726    Patent Document 7: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 55-64550    Patent Document 8: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 55-64551    Patent Document 9: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 56-10144    Patent Document 10: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 59-106436    Patent Document 11: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 63-41432 (corresponding to U.S. Pat. No. 4,661,609)    Patent Document 12: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 63-238043    Patent Document 13: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 64-31737 (corresponding to U.S. Pat. No. 4,691,041)    Patent Document 14: U.S. Pat. No. 4,734,518    Patent Document 15: U.S. Pat. No. 3,803,201    Patent Document 16: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 4-198141    Patent Document 17: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 4-230243    Patent Document 18: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-213830 (corresponding to German Patent No. 4,129,316)    Patent Document 19: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 6-9507 (corresponding to German Patent No. 4,216,121)    Patent Document 20: International Publication No. WO 97/23445    Patent Document 21: International Publication No. WO 00/51954    Patent Document 22: Unexamined Japanese Patent Application Laid-Open Specification No. 2002-308804    Patent Document 23: Unexamined Japanese Patent Application Laid-Open Specification No. 2004-131394